1. Field of the Invention
The present invention relates to gas composition sensors for sensing the air-fuel ratio of the exhaust gases from automobiles, for example.
2. Description of the Prior Art
Recently, in connection with the control of objectionable emissions from internal combustion engines, gas composition sensors have been in use as a means of sensing the air-fuel ratio of mixtures supplied for burning in an engine. In other words, where an exhaust gas purifying catalyst is employed as a means of controlling objectionable emissions from an engine, the air-fuel ratio of mixtures must always be maintained at a proper value in order to allow the catalyst to give full play to its capacity. However, with the carburetors of the ordinary engines or with the fuel injection systems of the fuel injection engines, even if the carburetor or the injection system is adjusted so that the air-fuel ratio of mixture is maintained at a predetermined value, the air-fuel ratio will in fact be varied greatly. As a result, in order that the air-fuel ratio of mixture may be maintained constant, the actual air-fuel ratio must be sensed by some means or other to feedback the resulting detection signal to the carburetor or the injection system.
The gas composition sensor senses the air-fuel ratio by utilizing the fact that the variation in the concentration of the exhaust gas composition is directly related closely to the air-fuel ratio of mixture.
In the past, to sense variation in the concentration of the composition of exhaust gases to be sensed, a gas composition sensing element comprising a transition metal oxide with a catalytic layer deposited on its outer surface has been used to sense the variation in the concentration by sensing the variation in the electric resistance value of the element, and the catalytic layer has the function of improving the sensitivity of the transition metal oxide to variation in the concentration of the exhaust gas composition. The use of such catalytic layer has been proposed by the present assignee by U.S. Patent Application of Hattori et al, Ser. No. 850,032, now abandoned, which was filed on Nov. 9, 1977.
Although the response of the gas composition sensing element to variation in the exhaust gas composition can be improved with decrease in the thickness of the gas composition sensing element, the strength of the gas composition sensing element will be reduced with decrease in its thickness, thus making it difficult to improve its response to an extreme extent.